The objective of the project is to investigate the relationship between leukemia and reticuloendothelial system (RES) function in the choloroleukemic rat. Research is being directed toward resolving the problems of (1) how and to what degree phagocytic activity is altered in leukemia and (2) whether alterations in phagocytosis are associated with modifications of macrophage (cellular) or opsonic (humoral) activity. The technique for perfusion of the isolated rat spleen is being used to assess blood clearance by the splenic RES of gelatin stabilized radioactive gold (Au198). The results to date indicate that there is no significant difference in the rate of clearance of Au198 between spleens of normal and leukemic rats perfused with blood collected from normal rats. When blood collected from leukemic rats is used as perfusate the clearance rate of both normal and leukemic spleens is significantly lower. These findings suggest that humoral factors are either present in leukemic blood which depress clearance or that stimulatory factors otherwise present in normal blood are deficient or absent.